1. Field of the Invention
The present invention relates to a method of adjusting a cathode operating temperature and to a writing apparatus. More specifically, for example, the present invention relates to a method of adjusting an operating temperature of a cathode of a beam source used in an electron beam writing apparatus.
2. Description of Related Art
In electron beam apparatuses, an electron gun is used as a beam source. As for the electron beam apparatuses, various apparatuses such as an electron beam writing apparatus and an electron microscope can be exemplified. With regard to an electron beam writing technology, for example, it intrinsically has excellent resolution and is used for producing a high-precision master pattern.
The lithography technique that advances miniaturization of semiconductor devices is extremely important since it is a sole process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. For forming a desired circuit pattern on such semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high accuracy is needed. Then, the electron beam writing apparatus is used for producing such a high-precision master pattern.
FIG. 7 is a conceptual diagram for explaining operations of a variable shaped electron beam writing or “drawing” apparatus. As shown in the figure, the variable shaped electron beam (EB) writing apparatus operates as described below. A first aperture 410 has a quadrangular opening 411 for shaping an electron beam 330. A second aperture 420 has a variable-shape opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shape opening 421 of the second aperture 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during the writing. In other words, a quadrangular shape that can pass through both the opening 411 and the variable-shape opening 421 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture 410 and the variable-shape opening 421 of the second aperture 420 is referred to as a variable shaped beam (VSB) system.
In order to enhance throughput of electron beam writing apparatuses, it is absolutely necessary to increase beam current density. Then, in order to realize a high current density, it is necessary to set the cathode temperature of an electron gun to be a high temperature. However, if the cathode temperature is set high, since the evaporation speed of the cathode material increases, the cathode tip shape changes during writing. Therefore, the high temperature setting has its own limit.
Regarding the current density, it is adjusted by controlling an emission current composed of electrons emitted from the cathode. In the conventional electron beam writing apparatus, the electron gun is controlled so that an initially set emission current may always be constant. If writing is performed by this control method under high current density conditions, the optimum cathode operating temperature for the initially set emission current changes due to the change of the cathode tip shape and the like (degradation of the cathode). Therefore, after the cathode deteriorates, the emission current becomes unstable when the initially set cathode operating temperature is kept to be used. Then, it becomes necessary, in such a state of the cathode, to newly acquire an optimum cathode operating temperature for obtaining a stable emission current. However, if the cathode temperature is changed, the current density also changes. Then, if the current density changes, the dose to the target object also changes, which causes a problem of degrading the writing precision of patterns. Therefore, it is necessary to maintain current density.
Then, in order to maintain a desired current density, the emission current needs to be adjusted each time according to the degradation state of the cathode. However, if the emission current is adjusted, since the optimum operating temperature of the cathode also changes, it becomes necessary to newly optimize the cathode temperature. Furthermore, if the cathode temperature is changed, the current density also changes again. Thus, in order to obtain a desired current density, it becomes necessary to optimize the emission current and the cathode temperature while repeatedly performing adjustment of the emission current and the cathode temperature each time according to the degradation state of the cathode. Therefore, there is a problem in that the adjustment takes time. Particularly, there is a problem in that optimization of the cathode temperature at the time of adjusting the emission current takes time.
The inventor of the present invention has proposed a technique of optimizing an emission current and a cathode temperature for a desired current density at the time of starting up the apparatus, by respectively measuring and plotting an optimum cathode operating temperature at each of various emission current values at the stage when the apparatus is started up and before the cathode deteriorates (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2010-62374). However, this plot data does not fit after the cathode deteriorates.